Download The chemistry and physics of interstellar ices

The chemistry and physics of
interstellar ices
Klaus Pontoppidan
Leiden Observatory
Kees Dullemond
(MPIA, Heidelberg)
Helen Fraser
(Leiden)
Ewine van Dishoeck
(Leiden)
Neal Evans
(Univ. of Texas)
Geoff Blake
(Caltech)
The c2d team
Cardiff, Jan ‘05
Known molecular reservoirs in dense clouds (cores)
Abundance
% of C
20%
% of N
-
3.3x10-4 2.6x10-4 23%
1x10-4
9%
50%
15%
-
Nitrogenbearing ice
3x10-5
<1%
<1%
18%
PAHs
10-7-10-6 <1%
10%
<1%
<1%
~95%
~18%
H2
Oxygenbearing ice
Carbon dust
Silicates
Gas-phase
CO
Other gasphase
molecules
Total
1
4x10-4
% of O
29%
~60%
Grain mantles as chemical reservoirs
Comets, planets
CH3OCH3, CH2CH3CN,…
Gas-phase reactions
Circumstellar environment
Primitive cloud
Mol. Cloud
T=10-15 K
n~105 cm-3
CO, O, N, H…
Freeze-out
Evaporation
Surface reactions
Bare grain surface
Mostly hydrogenation
H2O, CH3OH,
CO2, NH3,…
Main questions
Formation of interstellar ices.
What forms first? Water? CO2?
What are the chemical pathways to
form the most abundant ice species?
How does the ice interact with the
gas-phase?
Evolution of ices
Which external processes are
important - UV, heating, energetic
particles?
What happens when prestellar ices
are incorporated into a protostellar
envelope and then a disk?
The big laboratory in the sky
Microscopic properties
Understanding astronomical ice absorption
spectra: Grain shape effects/distribution
of ices within a grain mantle + intermolecular interactions
Macroscopic properties
Distribution of ices in a
cloud/envelope/disk.
Dust temperatures, radiation fields,
density and history of the above
parameters.
Spectroscopy of ices
VLT-ISAAC 3-5 micron mode
H2O, CO, CH3OH, OCN-, (NH3) --- ~50 lines of sight
Spitzer-IRS 5-20 micron
H2O, NH4+, CH4, (NH3), (CH3OH), --- ~100 lines of sight
CO2
ISOCAM-CVF 5-16 micron
H2O, NH4+, CO2
Single line of sight
Traditional method
of observing interstellar
ices. Problem: almost
impossible to couple the
ice to the physical
condition of the cloud
Multiple embedded lines of sight
Good: Direct spatial
information can be
obtained.
Sources are bright.
Bad: Sources may interact
With the ice on unresolved
scales
Multiple background stars
Good: Unbiased ice spectra. Bad: Stars are faint in the mid-IR
SVS 4 - a cluster embedded in the outer envelope
of a class 0 protostar.
2MASS JHK
SMM 4
SVS 4
Pontoppidan et al 2003, 2004 A&A
Mapping of ice abundances
SMM 4
ISOCAM 6.7 micron
SCUBA 850 micron
(used to extract temperature+density profiles)
Most of the stars in SVS4 have very little IR excess: Extinction estimates are accurate
H2O ice
Both H2O and CH3OH
ices show a sudden
jump in abundance
At densities of
4x105 cm-3 and
1x105 cm-3, resp.
CH3OH ice
-The formation of
water seems to depend
on density.
-Methanol in high
abundance is very
localised.
CO ice seems to be divided
into two (or three) basic
components
CO+H2O
Pure CO
Pontoppidan et al. 2003, A&A, 408, 981
CO ice is mobile
< 10 K
10-20 K
30-70 K
Collings et al 2003
Pontoppidan et al. 2003, A&A
15.2 micron CO2 bending mode with Spitzer
Cold core
Envelope?
Large disk?
Ices in the Oph-F core
CRBR 2422.8-3423
(+) indicates an observed line of sight.
Pontoppidan et al. 2005, in prep
Radial map of CO and CO2 ices
Density
Spitzer-IRS
VLT-ISAAC
ISOCAM-CVF
NH4+
The formation of ice mantles can be directly modeled.
However, an accurate temperature-density model of
The core is required for accurate age estimates.
50%
T0 x 10
(equilibrium)
T0 x 3
T0
5%
Robert Hurt, SSC
CRBR 2422.8-3423 model
2D Monte Carlo model to compute temperature + density
structure of disk and envelope using JHK/(sub)mm imaging +
2-40 micron spectroscopy
90 AU flared disk (solar nebula style)
+
envelope/foreground material producing extinction
to account for the near-infrared colours.
Vary parameters by hand (a full grid would
take years to compute).
Comparison between observed JHKs composite and model
of CRBR 2422.8-3423
ISAAC JHKs
Model JHKs
Pontoppidan et al. 2005, ApJ, in press
Model fit to the SED of CRBR 2422.8-3423
30”
10”
Heated ice bands toward CRBR 2422.8-3423
H2O+’6.85 micron’ bands
Conclusion:
Most of the ice, in
particular the CO ice is
Not located in the disk, in
this case.
However, the NH4+ band
Shows evidence for
strong heating, requiring a
significant part of this
component to be located
in the disk.
Summary
• Different methods of observing interstellar
Ices:
1) Single line of sight toward embedded source.
2) multiple lines toward embedded and background stars.
3) disk ices coupled with a radiative transfer model.
• Examples given:
1)
2)
3)
4)
CRBR2422.8-3423 (disk)
SMM 4 (protostellar envelope)
Oph-F (dense core)
L723 (isolated dense core)
• Ices are important both for tracing the chemistry
and physical conditions of dense clouds…